A multi fuel stove for water heating?

[Jacob]

I'm talking of a radiator system from the multi fuel, not a thermostatically controlled boiler. The original Dowling page said it would have to be pumped - I'm wondering why that would be OK,

Because gravity fed heating would be very difficult to design, especially in a bungalow (but not impossible)

when pumped hot water wouldn't be. ....

Because at least part of the system has to be gravity fed as a safety measure and the hot water side is simpler. As far as I understand it!

 

[custard]

I wanted to have a log burner that also heated up the water. The builder talked me out of it and persuaded me to just get a stand alone log burner.

His argument was this, a log burner that was big enough for a hot water system would be way too powerful for an average sized room, because there's no effective way of turning down a log burner you'd just overheat the room and bake. And log burners aren't brilliant for very large rooms because the heat intends to be too localised. So either way having a log burner deliver hot water is a compromised solution...or so he said!

 

[Hot stuff]

I had a large Coalbrookdale wood burner years ago with a back boiler that heated all the radiators in the house purely by convection. I fitted a pump just in case when I plumbed it all in because I wasn't sure if it would work or not but never needed it.
Your builder was right though, it was very mild in the living room with it on. I remember having to open windows to cool things down a bit. The aga in the kitchen heated the hot water in a similar fashion but we needed to use a towel rail in the bathroom to regulate it or the water boiled in the immersion tank.
Not the most efficient ways of doing things but certainly the most reliable and easily maintained.

 

[Phil Pascoe]

Mine's 5kw to heat, 3kw to water, which is ideal for the size of room - but yes, he made sound points. I got around the localised heat in my old house by putting a 10" desk fan on the hearth blowing across the front and up the room. At the other end I fitted a 4" bathroom fan under the coving, blowing out into the stair well. In the winter when the stove was fired up 24/7 these two together made a big difference. Had I stayed I would have replaced the 4" fan with a 6" one.

 

[morturn]

I would have thought that you could use a direct combination cylinder as a simple setup. You should be able to pick one up for under £200. You could either put this on the ground floor, as long as the cylinder is higher than the boiler. Most important is that the circs from the boiler are set with a slight fall towards the boiler otherwise it will not work.

Theoretically, you could fit a coil to a direct cylinder too, and then run a few radiators off that with a pump.

Most combination cylinders now sold have sprayed on insulation so fitting in the loft should not really be a problem.

You cannot use a direct combination cylinder and a pump with an unregulated heat source, you must have a heat leak (radiator) in a room above the boiler if the pump fails. In you case in a bungalow, there is no place for the heat leak.

One final point, the dynamics of burning solid fuel and wood are quite different. Wood needs to burn a higher temperature with a secondary air source to be efficient. It also need a fire brick hearth to hold the heat in when the air is turned down. Wood stoves are never very efficient when they are used to heat water because of this. There are a few that work in a fashion, but they are better as a standalone heat source.

 

[Phil Pascoe]

From what I gather the cylinder would have to have a 28mm (or even possibly 35mm :shock: ) coil, as this is the size the plumbing has to be - which counts out any retro fitting.

 

[Jacob]

.....
One final point, the dynamics of burning solid fuel and wood are quite different. Wood needs to burn a higher temperature with a secondary air source to be efficient. It also need a fire brick hearth to hold the heat in when the air is turned down. Wood stoves are never very efficient when they are used to heat water because of this. .....

Gasification boiler.
NB anything with fire bricks and/or cast iron tends to be high maintenance, with burnt out bits needing regular replacement. Welded steel much less so.

 

[RogerS]

Hi - this is something I've looked at in some detail in the past. An uncontrolled heat source (which is what we are talking about), must be able to "dump" a very significant volume of heat (to avoid heating any water above boiling point) at all times, regardless of the presence of electricity i.e. it must demonstrably "fail safe" not " fail dangerous".

On our multifuel, back-boilered, Rayburn we have a an upstairs tank, gravity-fed by large 28mm pipe runs, not vertically above, but to one side with swept bends and sloping pipework for efficient thermo-syphoning. We also - per the manufacturers instructions - have a decent-sized rad (in the bathroom) permanently in this loop. We also have a small electrical pump feeding a couple of extra rads (the rads are on a physically separate, but hydraulically linked circuit using a special kind of tee valve the name of which escapes me). This works well - but I wouldn't want to feed this set-up with wood (I'd get no rest) - coal keeps me busy enough!

So, you can use pumped circulation as part of your solution, but you do need a large body of water to buffer large amounts of heat if you and/or electrons are not available to assist in regulating the system. It seems difficult to do this safely without some kind of gravity-fed tank (which I suppose doesn't need to be more than a foot or two above the boiler).

Cheers, W2S


PS I remember, when reading about thermal stores, that a tank can be physically alongside the boiler - just as long as there's a proper thermo-syphon linking the two.

Large thermal store could be an answer; the system instead of being able to dump heat as a safety measure, has enough storage in the system to absorb worst case over-heating.
This also means you can do batch burning i.e. smaller charges of fuel burnt fast with plenty of air, which is more efficient than the opposite, but might over-heat a more conventional set up
So double benefit; efficient burning and efficient heat storage

The correct term is an accumulator. Give these guys a call http://www.dunsterheat.co.uk/about.html They are down in the South West and know what they are talking about.

 

[RogerS]

.....
One final point, the dynamics of burning solid fuel and wood are quite different. Wood needs to burn a higher temperature with a secondary air source to be efficient. It also need a fire brick hearth to hold the heat in when the air is turned down. Wood stoves are never very efficient when they are used to heat water because of this. .....

Gasification boiler.
NB anything with fire bricks and/or cast iron tends to be high maintenance, with burnt out bits needing regular replacement. Welded steel much less so.

Spot on, Jacob. These boilers are brilliant...something like 90-95% efficient and the ash only needs emptying about every three weeks.

Couple of good Youtube videos and I want one of those logsplitters in the second video !

https://www.youtube.com/watch?v=TJaveSG6W28

https://www.youtube.com/watch?v=G9yh80wEaxY

And you can get money back from the Govt.

 

[Bod]

Back boiler in a multi-fuel or wood burner, heating hot water and rads, done regularly on narrow boats.
No big secret,all done by gravity. Heat rises.
The water in the primary circuit is not pressurised, there is a header tank, that is open to atmosphere, to allow for expansion.
The pipe work is done in 28 mm copper, and rises from the boiler at a gentle slope, to allow for the rising heated water, NO valves can be fitted.
The return pipe also falls to the boiler.
The closer the hot water tank is to the boiler, the better, less space for heat to be lost. The hot water tank should also be vented for safety.
It is better to use a small fire, so that should it run away then the system will stay safe.
You would have say, 8kw of rads, on a 5kw fire.
Normally on a boat, the boiler runs rads, by thermo syphon effect, and the hot water tank is heated by pumped a circuit from the 28 mm pipes.
There is no reason why the first rad, couldn't be the hot water tank, and the whole system work without any power input.

Bod

 

[Phil Pascoe]

I don't need rads, though I know there has to be heat sink of some description somewhere in the system. My problem basically is that I'm being told that although I have a six foot vertical pipe straight off the boiler, because it then drops four feet about eight feet away that gravity won't work or be safe and that a pump is a no no.

 

[Jacob]

I don't need rads, though I know there has to be heat sink of some description somewhere in the system. My problem basically is that I'm being told that although I have a six foot vertical pipe straight off the boiler, because it then drops four feet about eight feet away that gravity won't work or be safe and that a pump is a no no.

Larger diameter pipe would help with a gravity driven circuit.

 

[Bod]

I don't need rads, though I know there has to be heat sink of some description somewhere in the system. My problem basically is that I'm being told that although I have a six foot vertical pipe straight off the boiler, because it then drops four feet about eight feet away that gravity won't work or be safe and that a pump is a no no.

Go across at a angle, it'll work.
Or provided the return pipe falls direct to the boiler, your up and down feed pipe would work, but not at its best.

Bod

 

[Phil Pascoe]

No, the return has to go up and down as well - it can't go at waist level across the hallway.
Jacob - the boiler has fittings for 28mm. pipework.

 

[Jacob]

No, the return has to go up and down as well - it can't go at waist level across the hallway.
Jacob - the boiler has fittings for 28mm. pipework.

Could be wrong but I think a gravity fed circuit has to have a continuous up gradient to the top followed by a continuous down gradient to the bottom, with no switch backs etc. The bottom can be lower than the boiler i.e. the boiler can be on the riser part of the circuit, not necessarily at the lowest point.
Even with 28mm in/outlet a larger pipe circuit connected would have less resistance (in theory anyway)

 

[Phil Pascoe]

So how does a radiator circuit work, then - the water goes along and up through a radiator then down, along and then up through a radiator then down and so on?

 

[Jacob]

So how does a radiator circuit work, then - the water goes along and up through a radiator then down, along and then up through a radiator then down and so on?

In a gravity circuit the feed circuit goes around but the rads feed off individually, like sow and piglets. Pumped circuit ditto but easier to design. Otherwise the rads would get progressively colder.

 

[Woody2Shoes]

I detect a possible source of confusion here - the word "gravity".

There are two common uses of the term "gravity feed" which aren't really interchangeable:

1) Putting a tank above the highest tap in the house to establish a "head of water" which will 'push' water out of the taps. Thus water is moved by gravity.

2) The thermosyphon, convection-based process which uses water to move heat out of a boiler to somewhere else - gravity plays a part in this, but really it's heat (not water in any useful way) that's moved - by convection.

Heat moved out of the boiler is stored in an "accumulator" (a pretty old-fashioned term) or a "thermal store" (a term more widely used in recent years, often designed to allow multiple sources of heat e.g. solar/oil/etc. to feed one store) - a large body of water. Heat can be got out of this store (to produce either hot water and/or space heating) by pumping water (or allowing it to flow from a header tank under gravity alone) through a heat exchanger and thence to taps or rads.

I think that the advice that Phil has received - that you can't sensibly have a down-slope in any part of your thermosyphon loop is sound, it needs a continuous upward flow (on the 'hot' branch at least, but arguably on both legs).

I hope I've helped to clarify things!

Cheers, W2S

PS It's easy to underestimate the amount of energy stored in a hod-full of coal or a basket-full of dry wood - enough to melt a large chunk of cast iron, for example. This energy has to be managed safely - which is why a large body of water (which can absorb a surprisingly large amount of energy before it boils) is needed as the primary destination of the boiler's direct output (in a failsafe 'primary' circuit).

 

[Jacob]

.....
2) The thermosyphon, convection-based process which uses water to move heat out of a boiler to somewhere else - gravity plays a part in this, but really it's heat (not water in any useful way) that's moved - by convection.......

Yes it is confusing but gravity is the driver - hot air or water is less dense than cold and gravity draws the cold down, displacing the warm upwards, in a circuit. Convection is the movement of hot air/water, not of heat alone.
Heat alone can be moved by radiation (through a vacuum or transparent media, air , glass etc) and conduction (through a solid), which involve no moving material.